One of the greatest concerns in such fields as virtual reality or robotics is to track accurately the motion of a moving object in certain surroundings. Several methods have been used to track the motion of a moving object, and widely used is, among others, the method of tracking the motion of an object by means of a scanning device that collects surrounding information. The surrounding information is defined as information on surroundings based on the distance to an obstacle that is within a detectable range of the scanning device.
Namely, the scanning device mounted on an object collects the surrounding information, and then evaluates location and position information of the device based on data of the distance to the collected surrounding obstacles. The location information includes data of the spot where the scanning device is located in any absolute coordinate system. The position information includes data of the amount by which the scanning device is rotated with respect to the absolute coordinate system. While moving, the scanning device continues to collect such pieces of information and evaluate the location information and the position information, and keeps on tracking the motion of the scanning device within its surroundings. In a technical sense, tracking the motion of the scanning device is identical to tracking the motion of the moving object.
A laser scanning device has been widely used as a scanning device for the aforementioned method. FIG. 1 generally explains the motion of the laser scanning device 5. As illustrated in FIG. 1, the laser scanning device 5 is provided with a rotating portion that is operated by a motor 6. The rotating portion rotates once over a period of time Δt. During the time period Δt, the laser scanning device 5 emits a laser, at a constant time interval Δts, sequentially to each of a plurality of the points in its surroundings, and collects the distance information for each of the points in its surroundings by calculating the time taken to turn back from the points to the device. Namely, the information, which is measured by the scanning device 5 during the period of time Δt, is collected at the constant time interval Δts and is classified by each of the points. The pieces of the surrounding information classified by each of the points are not measured at the same time, but are collected at different times, i.e. with the time interval Δts.
If the laser scanning device 5 does not move but stays fixed at a spot, the distance from the device 5 to its surroundings will be constant even though the information for the points are measured at different times. In this case, the surrounding information obtained from the distance data of the surroundings will be identical to the information on the actual surroundings. That is, the motion of the laser scanning device 5 will be precisely calculated only from the surrounding information intermittently obtained in the surroundings of the device. However, as the scanning device, i.e. the laser scanning device 5, is continuously moving while scanning, the surrounding information measured during the period of time Δt, as stated above, are collected at different times with respect to each of the points of the surroundings. Thus, the surrounding information gathered during one cycle of scanning process is distorted, being different from the actual shapes of the surroundings. This causes the laser scanning device 5 not to track accurately the location and position of the device in its surroundings.
FIGS. 2 to 4 are conceptual views illustrating the surrounding information collected by a scanning device moving around.
FIG. 2 represents the outline of the surroundings 1 as a solid line. The scanning device is shown as a dot in the middle, and moves following the arrow in the figure. As shown in FIG. 3, when the scanning device moves in a direction of the arrow for the period of time Δt corresponding to one scanning cycle, the surrounding information 2, which has approached toward the device by the amount of distance through which the device moved, will be identified by the scanning device. Because the initial surroundings 1 do not change its shape, it is to be assured that the scanning device keeps on identifying the surrounding information as the initial surroundings 1.
However, as stated above, the scanning device collects the surrounding information with the time interval Δts during the period of time Δt, i.e. the scanning device collects the surrounding information while moving, and thus the scanning device identifies distorted surrounding information 3, as illustrated in FIG. 4. As such, determination of the present location and position information based on the distorted surrounding information 3 leads to an inaccurate detection of the motion of the scanning device. This problem gets worse as the time interval Δts and/or the velocity of the scanning device increase(s).